The present invention relates to recording apparatus. More particularly it relates to a fail safe shutdown circuit for a recording apparatus.
In the recording art, there has been provided a form of recording wherein a photosensitive recording medium is exposed to light images to produce latent images thereon. Those latent images are then developed by the application of heat to the photosensitive emulsion bearing the latent images. In one form of such medium, a photosensitive emulsion is carried on one surface of a base member while an electrically conductive backing coating is applied to the other surface of the base member. After exposure to the light images, the recording medium is drawn across a plurality of electrodes with the electrodes engaging the conductive backing on the record medium. The electrodes are energized with electrical energy to pass electrical current through the conductive backing. That electrical current is controlled to provide the desired heating to effect the proper development of latent images on the emulsion side of the record medium. Inasmuch as the record medium may be driven at any of a plurality of selected speeds, it is apparent that the required current will of necessity be varied in accordance with the selected speeds in order to provide the required heating effect to produce a desired development.
If too much energy is applied to the record medium, especially at low speeds, the record medium may catch fire, due to the excessive heat in the conductive backing. The temperature of the backing is very difficult to sense directly. The thermal mass of the record medium is so small that if anything actually touches the record medium it will be cooled slightly in that area and the image will not develop as fully as the rest of the image. This will leave an undeveloped streak on the record. In previous efforts to accomplish the desired result, a very small thermistor was mounted very near but not touching the record member. This method has not proved entirely satisfactory for several reasons.
In order to respond quickly enough to prevent a fire, approximately four seconds, the thermistor must be very small and is therefore very fragile. It must be suspended in air by its tiny leads whereby the surrounding surfaces will not sink the heat away from it. Since it is uncovered or exposed when a record member is being loaded into the apparatus, it must also be protected from damage by operators fingers. Such protection is inconsistent with the thermistor being close to the record member. If the thermistor is not close to the record member, it will not respond quickly enough to prevent a fire.
The sensing circuit which detects the thermistors temperature must be adjusted very precisely. If the limiting temperature is set too high, the paper will catch fire before the safety circuit is actuated. If the limiting temperature is set too low, the safety circuit will be actuated whenever the record member is run at low speed for a long period of time. Nuisance tripping such as this, is of course not acceptable. Additionally, the correct setting of the thermistor and the accompanying circuit involves considerable setup time by production technicians and wastes a significant amount of the expensive record medium, due to the long runs at low speeds which are necessary in order to assure a proper setup.
Further, the thermistor bead is sensitive to debris. When the system has been operated for any significant length of time, small particles may accumulate on the thermistor bead and will partially insulate it and thus slow the response time.
Finally the thermistor circuit, as explained above, was not fail-safe and was easily subject to human error.